This section provides background information related to the present disclosure which is not necessarily prior art.
FIG. 1 shows a typical prior art single module UPS system 100. The basic elements of UPS system 100 are rectifier 102, inverter 104, output transformer 106, a backup DC power source 108, and a controller 110. UPS system 100 also includes a bypass switch (not shown). An input of rectifier 102 is coupled to a source of AC power (not shown). An output of rectifier 102 is coupled to a DC bus 112. An input of inverter 104 is coupled to DC bus 112. An output 105 of inverter 104 is coupled to a primary side 114 of output transformer 106. A secondary side 116 of output transformer 106 is coupled to output 118 of UPS system 100. A Grass filter circuit 120 is coupled to the secondary side 116 of output transformer 106. A filter circuit 122 is coupled to the primary side 114 of output transformer 106.
Controller 100 controls UPS system 100 including controlling inverter 104 by varying the duty cycle of the switching devices in inverter 104 so that inverter 104 provides a desired output voltage. In this regard, controller 110 has inputs 124 and output 126. Inputs 124 include inputs coupled to current transformers CT that sense currents in various parts of UPS 100 such as shown in FIG. 1, including a load current flowing through output 118 of UPS system 100, and voltage sensors VS that sense voltage such as a primary side voltage at primary side 114 of output transformer 106 (if primary side voltage control is being used as discussed below) or a secondary side voltage at secondary side 116 of output transformer 106 (if secondary side voltage control is being used as discussed below).
FIG. 2 shows a simplified output equivalent circuit diagram for UPS system 100. Resistive/inductive (RL) load is coupled to output 118 of UPS system 100. As shown in FIG. 2, in secondary side voltage control, the voltage control reference point is the voltage at secondary side 116 of output transformer 106. As also shown in FIG. 2, in primary side voltage control, the voltage control reference point is the voltage at primary side 114 of output transformer 106.
In many cases, secondary side voltage control of a single module UPS system of the type shown in FIG. 1 is used to get fast transient response and good steady state voltage control. Secondary side voltage control can result in instability issues for single module UPS systems when certain types of loads are being powered by the UPS system, such as high lighting loads. It is also difficult to stabilize multi-module UPS systems using secondary side voltage control. A multi-module UPS system is a UPS system that has multiple UPS modules coupled in parallel. The UPS modules share power providing that the power demanded by the load(s) coupled to the UPS system is equally distributed among the UPS modules.
Primary side voltage control is often used in single module UPS systems having loads of the type that result in instability issues. Primary side control is also often used in multi-module UPS systems as it is difficult to stabilize multi-module UPS systems using secondary side voltage control. By moving the voltage reference point to the primary side of the output transformer, extra impedance is added (i.e. the output transformer) between the voltage control reference point (the primary side of the output transformer) and the load coupled to the output of the UPS system, which makes it easier to stabilize the UPS system and have it perform well.
However, the impedance provided by the output transformer introduces another problem in that the system output voltage will vary with different types of loads since the current will change according to the load, as shown in FIGS. 3A and 3B. For primary side voltage control, FIGS. 3A and 3B show that different load conditions will lead to different current in the space vector. The voltage across the output transformer is VT=IT*jωXT (where jωXT is the impedance of output transformer 106 and IT is the current flowing through output transformer 106). The voltage at the output 118 of UPS system 100 is Vload=Vout−VT (where Vout is the output voltage of inverter 104). Since a capacitive load causes leading current IT, this results in the space vector Vload shown in FIG. 3A. Since an inductive load causes lagging current IT this results in the space vector Vload as shown in FIG. 3B. As can be seen from FIGS. 3A and 3b, with a fixed inverter output voltage Vout, Vload resulting from a capacitive load is bigger than Vload resulting from an inductive load, and they will vary with different load sizes. As can be seen from the above, because the current flowing through the output transformer 106 changes with different load conditions, the voltage drop across the output transformer 106 also changes accordingly. This causes the output voltage of the UPS system at output 118 to vary with different types of loads.
Though the equations Vload=Vout−VT and VT=IT*jωXT are quite simple to understand, complex number operations are always complicated to implement in a digital signal processor (DSP) and will occupy considerable chip resources. Also, current values from current measuring devices are real numbers and it takes more chip resources to convert those real numbers to complex numbers in order to do complex number operations. Further, the equivalent impedance XT of the output transformer is not a straightforward inductance value. Considering all the above, it's not very practical to use the equations Vload=Vout−VT and VT=IT*jωXT to dynamically calculate VT in UPS systems.